Conventionally, there has been provided a vehicle obstacle detection device which detects, with sound waves or ultrasonic waves, an obstacle around a vehicle (e.g., see WO 2009/119577 A1). The document discloses the vehicle obstacle detection device which detects the obstacle around the vehicle, with a sensor transmitting and receiving the ultrasonic waves, thereby assisting parking of the vehicle. In the vehicle obstacle detection device described in the document, the sensor transmits transmission waves, and receives reflected waves that are reflected by an obstacle, and then the vehicle obstacle detection device detects the obstacle around the vehicle, based on a time period between a timing of when the sensor transmits the transmission waves and a timing of when the sensor receives the reflected waves. The sensor of such a vehicle obstacle detection device generally includes a piezo element.
This kind of vehicle obstacle detection device receives power supply from a battery that is installed in the vehicle. The power supply from the battery is also used for other apparatuses that are installed in the vehicle, and accordingly the voltage fluctuates, depending on use situations of other apparatuses. Here, because the sensor including the piezo element outputs sound waves or ultrasonic waves with a sound pressure depending on the supplied voltage, the output sound pressure is changed together with the fluctuation of the supplied voltage. Therefore, the conventional vehicle obstacle detection device keeps constant the output of the transmission waves by the sensor, using a stabilization power supply circuit that keeps constant the voltage to be supplied to the sensor.
However, an output voltage becomes lower than an input voltage, due to using the stabilization power supply circuit. Accordingly, when using the stabilization power supply circuit to keep constant the voltage to be supplied to the sensor, there is a case where the output sound pressure from the sensor becomes smaller than a desired sound pressure. In this case, there is a problem that a distance at which the vehicle obstacle detection device can detect the obstacle is reduced.